Balancing circuit for a metal detector

ABSTRACT

A balancing circuit for a metal detector. The metal detector includes an oscillating power source, a transmit coil connected to the oscillating power source, first and second receive coils inductively coupled to the transmit coil, a first amplitude balancing circuit connected to the first receive coil, and a first phase balancing circuit connected to the first receive coil. The first phase balancing circuit includes a capacitor and a variable resistor.

BACKGROUND

The invention relates to a balancing circuit for a metal detector. Knownmetal detectors include an oscillator (or transmit) coil and twodetector (or receive) coils. The receive coils are typically positionedon either side of the oscillator coil and substantially equidistanttherefrom. The oscillator coil is provided with an oscillatory signalthat is inductively coupled to the detector coils. Metal passing throughthe coils causes an imbalance in the inductively coupled signals, whichcan be used to detect the presence of the metal.

To operate properly, the inductively coupled signals should be balancedsuch that the signals produced by each receive coil are the same whenmetal is not present. Due to variances in tolerances, aging ofmaterials, etc., it is necessary to provide a means of balancing theinductively coupled signals when no metal is present.

SUMMARY

In one embodiment, the invention provides a metal detector including anoscillating power source, a transmit coil connected to the oscillatingpower source, first and second receive coils inductively coupled to thetransmit coil, a first amplitude balancing circuit connected to one ofthe first receive coil and the second receive coil, and a first phasebalancing circuit connected to one of the first receive coil and thesecond receive coil, the first phase balancing circuit including a firstcapacitor and a first variable resistor.

In another embodiment, the invention provides a method of balancing ametal detector. The metal detector includes a transmit coil, first andsecond receive coils inductively coupled to the transmit coil, adifferential circuit coupled to the first and second receive coils, afirst amplitude balancing circuit coupled to one of the first receivecoil and the second receive coil and including a first variableresistor, and a first phase balancing circuit coupled to one of thefirst receive coil and the second receive coil and including acapacitor. The method includes detecting a difference between a signalfrom the first receive coil and a signal from the second receive coilwhen there is no material in the metal detector, adjusting the firstvariable resistor to reduce the difference, adjusting the secondvariable resistor to reduce the difference, and repeating adjustment ofthe first and second variable resistors to reduce the difference.

In another embodiment the invention provides a signal adjuster for aninductor. The signal adjuster includes, an amplitude adjuster configuredto provide a resistance between a lead of the inductor and ground, and aphase adjuster configured to provide a capacitance and a variableresistance between a lead of the inductor and ground. The variableresistance is configured to adjust a phase of a signal generated by theinductor.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a prior art metal detector.

FIG. 2 is a schematic diagram of a construction of a balancing circuitaccording to the invention.

FIG. 3 is a partial schematic diagram, partial block diagram of aconstruction of a metal detector according to the invention.

FIG. 4 is a flow chart of a first embodiment of a process for balancingthe metal detector of FIG. 3.

FIGS. 5A and 5B are a flow chart of a second embodiment of a process forbalancing the metal detector of FIG. 3.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

In addition, it should be understood that embodiments of the inventioninclude hardware, software, and electronic components or modules that,for purposes of discussion, may be illustrated and described as if themajority of the components were implemented solely in hardware. However,one of ordinary skill in the art, and based on a reading of thisdetailed description, would recognize that, in at least one embodiment,the electronic based aspects of the invention may be implemented insoftware. Similarly, some embodiments of the present invention describedherein operate utilizing software. One of ordinary skill in the art, andbased on a reading of this detailed description, would recognize that,in at least one embodiment, such embodiments could instead operatewithout software, instead utilizing electronic circuitry and otherhardware configured to perform the same functions. As such, it should benoted that any number and combination of hardware-based devices,software-based devices, and structural components may be utilized toimplement the various embodiments of the present invention. Also,although various components of the present invention are described andillustrated herein as being defined by modules, it will be appreciatedthat the modules described and illustrated herein can be configured in asignificantly different manner, can be defined by one or more othermodules performing additional tasks, and/or can be defined by fewermodules.

FIG. 1 schematically illustrates a construction of a prior art metaldetector 100. The metal detector 100 includes a passageway 105. Atransmit coil 110 is centrally positioned around the passageway 105. Afirst receive coil 115 and a second receive coil 120 are positionedaround the passageway 105 on opposite sides of the transmit coil 110,substantially equidistant from the transmit coil 110. A first lead 125of the first receive coil 115 and a first lead 130 of the second receivecoil 120 are connected to ground. A second lead 135 of the first receivecoil 115 and a second lead 140 of the second receive coil 120 areconnected to a differential circuit 145.

During operation of the metal detector 100, an oscillatory signal 150(e.g. an AC signal) is supplied to the transmit coil 110. The transmitcoil 110 transmits a signal, based on the oscillatory signal. The firstreceive coil 115 and the second receive coil 120 receive, via inductivecoupling, the signal transmitted by the transmit coil 110. The first andsecond receive coils 115 and 120 each generates an output signal basedon the signals they receive from the transmit coil 110. When there is nomaterial, particularly no metallic material, in the passageway 105(i.e., the passageway 105 is empty), the signals received by, and outputby, the first and second receive coils 115 and 120 should besubstantially equivalent. The differential circuit 145 compares theoutput of the first receive coil 115 to the output of the second receivecoil 120, and outputs a signal indicative of a difference in thesignals. In the case, where there is no material in the passageway 105,the signals should be substantially equivalent, and the differentialcircuit 145 outputs a signal with a zero or near zero signal (e.g, azero amplitude analog signal or a digital zero value).

When a non-metal material (e.g., a food product) enters the passageway105, the signals received by the first receive coil 115 and the secondreceive coil 120 differ. The amplitude of these signals can differsignificantly but the phase angles of the signals will generally bedifferent than the phase angle generated by a metal. Therefore, theoutputs of the first receive coil 115 and the second receive coil 120,amplitude and phase angle, will differ. The differential circuit 145then outputs a signal (e.g., analog or digital) indicative of thedifference between the first receive coil 115 output and the secondreceive coil 120 output.

When a piece of metal (ferrous or non-ferrous) enters the passageway105, the metal distorts the signal transmitted by the transmit coil 110,and therefore, the signals received by the first receive coil 115 andthe second receive coil 120. The distortion is greater nearer the metal.Therefore, the receive coil nearer the metal receives a signal having agreater distortion than the receive coil that is farther away from themetal. Accordingly, the outputs of the first receive coil 115 and thesecond receive coil 120 can differ relatively significantly when metalis present in the passageway 105. The differential circuit 145 receivesthe different signals from the first receive coil 115 and the secondreceive coil 120 and generates an output (e.g., analog or digital)indicative of the difference. The metal detector 100 receives therelatively large output of the differential circuit 145 and detects thepresence of metal in the passageway 105, taking appropriate action(e.g., sounding an alarm, stopping a conveyor, etc.).

The signals received by, and output by, the first receive coil 115 andthe second receive coil 120, should be equivalent when there is nomaterial in the passageway 105. Thus, the differential circuit 145should output a zero signal when there is no material in the passageway105. However, differences in the signals output by the first and secondreceive coils 115 and 120 can occur, even when there is no material inthe passageway 105. For example, variances in the tolerances of thefirst receive coil 115 and the second receive coil 120, as well as inmanufacturing the metal detector 100 (e.g., the positioning of thereceive coils 115 and 120 relative to the transmit coil 110), can resultin differences in the signals received by, and output by, the receivecoils 115 and 120. In addition, changes in temperature can also resultin differences in the signals received by, and output by, the receivecoils 115 and 120.

The signals output by the receive coils include two components thatshould match in order for the differential circuit 145 to output arelatively zero signal. The components are (1) the amplitudes of thesignals and (2) the phases of the signals. Reducing and/or removing thevariations in the amplitude and/or phase of the outputs of the firstreceive coil 115 and/or the second receive coil 120, when no material ispresent in the passageway 105, improves the ability of the metaldetector 100 to detect when metal is actually present in the passageway105.

A balancing circuit can be used to adjust the outputs of the firstreceive coil 115 and the second receive coil 120, such that theamplitude and phase of the output signals match when there is nomaterial in the passageway 105. FIG. 2 illustrates a construction of abalancing circuit 200. The balancing circuit 200 includes an amplitudebalancing circuit 205 and a phase balancing circuit 210. The amplitudebalancing circuit 205 includes a variable resistor 215 (e.g., a 0-20 k Ωmodel AD5262 manufactured by Analog Devices) having a wiper 220connected to a receive coil 225 and a second lead 230 connected toground.

The phase balancing circuit 210 includes a capacitor 235 (e.g., 2 to 20nanofarad), a variable resistor 240 (e.g., a 0-20 k Ω model AD5262manufactured by Analog Devices), and a fixed resistor 245 (e.g., 100Ω).A first lead 250 of the capacitor 235 is connected to the receive coil225. A second lead 255 of the capacitor 235 is connected to a wiper 260of the variable resistor 240. A second lead 265 of the variable resistor240 is connected to ground. The fixed resistor 245 is connected acrossthe variable resistor 240.

FIG. 3 illustrates a construction of a metal detector 300 embodying thepresent invention. The metal detector 300 includes an oscillatory powersource 305, a transmit coil 315, a first receive coil 320, a secondreceive coil 325, a first balancing circuit 330, a second balancingcircuit 335, a receiver circuit 340, an analog-to-digital (A/D)converter 345, a controller 350, and an indicator 355.

The oscillatory power source 305 provides an oscillatory signal (e.g.,an AC signal) to the transmit coil. The transmit coil 315 transmits asignal which is received by the first receive coil 320 and the secondreceive coil 325. The first receive coil 320 and the second receive coil325 provide output signals, to the receive circuit 340, based on thesignals they receive from the transmit coil 315. The receive circuit 340amplifies any imbalance in the signals received from the first receivecoil 320 and the second receive coil 325. The amplified differentialsignal is provided to the A/D converter 345 where it is converted into adigital value indicative of the amplitude of the amplified differentialsignal. The digital value is then provided to the controller 350, whichis also connected to the indicator 355, and the first and secondbalancing circuits 330 and 335. The controller 350 reduces and/orremoves differences between the output signals (i.e., balances thesignals) of the first receive coil 320 and the second receive coil 325,when there is no material present in a passageway 380, by adjusting oneor more of an amplitude balancing circuit 360 and a phase balancingcircuit 370, of the first balancing circuit 330, and an amplitudebalancing circuit 365 and a phase balancing circuit 375, of the secondbalancing circuit 335.

FIG. 4 is a flow chart of an embodiment of a balancing operation 400 forthe metal detector 300. The controller 350 begins by adjusting the firstamplitude balancing circuit 360 (block 405) (e.g., adjusting theresistance of a variable resistor 410). The controller 350 monitors theamplified differential signal received from the A/D converter 345 andadjusts the first amplitude balancing circuit 360 until an amplitude ofthe differential signal is as low as can be achieved by adjusting thebalancing circuit 360.

Next the controller 350 adjusts the first phase balancing circuit 370(block 415) (e.g., adjusting the resistance of a variable resistor 420).The controller 350 monitors the amplified differential signal receivedfrom the A/D converter 345 and adjusts the first phase balancing circuit370 until the amplitude of the differential signal is as low as can beachieved by adjusting the balancing circuit 370. The controller 350 thendetermines if adjusting the first amplitude balancing circuit 360 and/orthe first phase balancing circuit 370 reduced the amplitude of thedifferential signal (block 425). If the amplitude of the differentialsignal was reduced, the controller 350 repeats the previous processbeginning at block 405 with adjusting the first amplitude balancingcircuit 360.

If the amplitude of the differential signal was not reduced, thecontroller 350 adjusts the second amplitude balancing circuit 365 (block430) (e.g., adjusting the resistance of a variable resistor 435). Thecontroller 350 monitors the amplified differential signal received fromthe A/D converter 345 and adjusts the second amplitude balancing circuit365 until an amplitude of the differential signal is as low as can beachieved by adjusting the balancing circuit 365.

Next the controller 350 adjusts the second phase balancing circuit 375(block 440) (e.g., adjusting the resistance of a variable resistor 445).The controller 350 monitors the amplified differential signal receivedfrom the A/D converter 345 and adjusts the second phase balancingcircuit 375 until the amplitude of the differential signal is as low ascan be achieved by adjusting the balancing circuit 375. The controller350 then determines if adjusting the second amplitude balancing circuit365 and/or the second phase balancing circuit 375 reduced the amplitudeof the differential signal (block 450). If the amplitude of thedifferential signal was reduced, the controller 350 repeats the previousprocess beginning at block 430 with adjusting the second amplitudebalancing circuit 365.

If the amplitude of the differential signal was not reduced, thecontroller 350 determines if the amplitude of the differential signalwas reduced while repeating blocks 440 and 450 (i.e., since lastadjusting the first amplitude balancing circuit 360 and/or the firstphase adjusting circuit 370). If the amplitude of the differentialsignal was reduced while repeating blocks 440 and 450, the controller350 repeats the whole process beginning at block 405 with adjusting thefirst amplitude balancing circuit 360. If the differential signal wasnot reduced, the receive coils are balanced, the process is complete,and the sensitivity of the metal detector 300 is maximized.

FIGS. 5A and 5B illustrate a flow chart of another embodiment of abalancing operation 500. The controller 350 begins by adjusting thefirst amplitude balancing circuit 360 (block 505) (e.g., adjusting theresistance of the variable resistor 410). The controller 350 monitorsthe amplified differential signal received from the A/D converter 345and adjusts the first amplitude balancing circuit 360 until an amplitudeof the differential signal is as low as can be achieved by adjusting thebalancing circuit 360. The controller 350 then determines if adjustingthe first amplitude balancing circuit 360 reduced the amplitude of thedifferential signal (block 510). If the amplitude of the differentialsignal was reduced, the controller 350 sets an amplitude flag to one(block 515). If the amplitude of the differential signal was notreduced, the controller 350 sets the amplitude flag to two (block 520).

Next the controller 350 adjusts the first phase balancing circuit 370(block 525) (e.g., adjusting the resistance of the variable resistor420). The controller 350 monitors the amplified differential signalreceived from the A/D converter 345 and adjusts the first phasebalancing circuit 370 until the amplitude of the differential signal isas low as can be achieved by adjusting the balancing circuit 370. Thecontroller 350 then determines if adjusting the first phase balancingcircuit 370 reduced the amplitude of the differential signal (block530). If the amplitude of the differential signal was reduced, thecontroller 350 sets a phase flag to one (block 535). If the amplitude ofthe differential signal was not reduced, the controller 350 sets thephase flag to two (block 540).

The controller 350 then checks the amplitude flag (block 545). If theamplitude flag equals one, the controller 350 adjusts the firstamplitude balancing circuit 360 (block 550) until the differentialsignal is as low as can be achieved by adjusting the balancing circuit360. If the amplitude flag equals two, the controller adjusts the secondamplitude balancing circuit 365 (block 555) (e.g., by adjusting thevariable resistor 435) until the differential signal is as low as can beachieved by adjusting the balancing circuit 365.

The controller 350 then checks the phase flag (block 560). If the phaseflag equals one, the controller 350 adjusts the first phase balancingcircuit 370 (block 565) until the differential signal is as low as canbe achieved by adjusting the balancing circuit 370. If the phase flagequals two, the controller adjusts the second phase balancing circuit375 (block 5705) (e.g., by adjusting the variable resistor 445) untilthe differential signal is as low as can be achieved by adjusting thebalancing circuit 375.

Next the controller 350 checks if any change in the amplitude of thedifferential signal was achieved by adjusting one of the amplitudebalancing circuits 360 or 365 and/or by adjusting one of the phasebalancing circuits 370 or 375 (block 575). If there was a change (i.e.,reduction) in the amplitude of the differential signal, the controller350 repeats the adjustment process beginning at block 545. If there wasno change, the process is complete and the receive coils are balanced.

The controller 350, as discussed above can be in the form of amicrocontroller or microprocessor and can include other components suchas a power supply, memory, an A/D converter, and filters. Further, it isenvisioned that components shown in the embodiments above can becombined and/or separated resulting in different arrangements of thecircuits.

The invention has been described in constructions and embodiments ofmetal detectors; however, the invention has application in other typesof metal detectors and other inductor systems requiring amplitude and/orphase adjustment.

The values of components above are given by way of example only anddifferent combinations and values of components (e.g., resistances andcapacitances) are contemplated in the invention.

Thus, the invention provides, among other things, a new and usefulbalancing circuit for a metal detector. Various features and advantagesof the invention are set forth in the following claims.

1. A metal detector, comprising: an oscillating power source; a transmitcoil connected to the oscillating power source; first and second receivecoils inductively coupled to the transmit coil; a first amplitudebalancing circuit connected to one of the first receive coil and thesecond receive coil; and a first phase balancing circuit connected toone of the first receive coil and the second receive coil, the firstphase balancing circuit including a first capacitor and a first variableresistor.
 2. The metal detector of claim 1, further comprising, a secondamplitude balancing circuit connected to the second receive coil; and asecond phase balancing circuit connected to the second receive coil, thesecond phase balancing circuit including a second capacitor and a secondvariable resistor; wherein the first amplitude balancing circuit iscoupled to the first receive coil and the first phase balancing circuitis coupled to the first receive coil.
 3. The metal detector of claim 2,further comprising a differential circuit configured to receive a signalfrom the first receive coil and a signal from the second receive coiland to output a signal indicative of the difference between the signalreceived from the first receive coil and the signal received from thesecond receive coil.
 4. The metal detector of claim 2, wherein the firstamplitude balancing circuit includes a third variable resistor and thesecond amplitude balancing circuit includes a fourth variable resistor.5. The metal detector of claim 4, further comprising a controllerconfigured to adjust the first, second, third, and fourth variableresistors such that the signals from the first receive coil and thesecond receive coils are substantially equivalent when there is nomaterial in the metal detector.
 6. The metal detector of claim 1,wherein the first capacitor and the first variable resistor areconnected in series, and the first variable resistor is also connectedto ground and the first capacitor is also connected to the first receivecoil.
 7. The metal detector of claim 6, further comprising a fixedresistor connected across the first variable resistor.
 8. The metaldetector of claim 2, wherein the first capacitor and the first variableresistor are connected in series, and the first variable resistor isalso connected to ground and the first capacitor is also connected tothe first receive coil, and the second capacitor and the second variableresistor are connected in series, and the second variable resistor isalso connected to ground and the second capacitor is also connected tothe second receive coil.
 9. The metal detector of claim 8, furthercomprising a first fixed resistor connected across the first variableresistor and a second fixed resistor connected across the secondvariable resistor.
 10. A method of balancing a metal detector having atransmit coil, first and second receive coils inductively coupled to thetransmit coil, a differential circuit coupled to the first and secondreceive coils, a first amplitude balancing circuit coupled to one of thefirst receive coil and the second receive coil and including a firstvariable resistor, and a first phase balancing circuit coupled to one ofthe first receive coil and the second receive coil and including acapacitor and a second variable resistor, the method comprising:detecting a difference between a signal from the first receive coil anda signal from the second receive coil when there is no material in themetal detector; adjusting the first variable resistor to reduce thedifference; adjusting the second variable resistor to reduce thedifference; and repeating adjustment of the first and second variableresistors to reduce the difference.
 11. The method of claim 10, whereinthe repeating of the adjustment of the first and second variableresistors continues until adjustment of the first and second variableresistors does not reduce the difference.
 12. The method of claim 10,wherein the metal detector includes a second amplitude balancing circuitcoupled to the second receive coil and including a third variableresistor, and a second phase balancing circuit coupled to the secondreceive coil and including a capacitor and a fourth variable resistor,wherein the first amplitude balancing circuit is coupled to the firstreceive coil and the first phase balancing circuit is coupled to thefirst receive coil, and wherein the method further comprises, adjustingthe third variable resistor to reduce the difference, and adjusting thefourth variable resistor to reduce the difference.
 13. The method ofclaim 12, further comprising, repeating adjustment of the third andfourth variable resistors to reduce the difference.
 14. The method ofclaim 12, wherein the repeating of the adjustment of the third andfourth variable resistors continues until adjustment of the third andfourth variable resistors does not reduce the difference.
 15. The methodof claim 12, further comprising, repeating adjustment of the first,second, third, and fourth variable resistors to reduce the difference.16. The method of claim 15, wherein the repeating of the adjustment ofthe first, second, third, and fourth variable resistors continues untiladjustment of the second, third, and fourth variable resistors does notreduce the difference.
 17. The method of claim 15, further comprising,repeating adjustment of one of the first and third variable resistorsand one of the second and fourth variable resistors until theadjustments do not reduce the difference.
 18. A signal adjuster for aninductor, comprising: an amplitude adjuster configured to provide aresistance between a lead of the inductor and ground; and a phaseadjuster configured to provide a capacitance and a variable resistancebetween a lead of the inductor and ground, the variable resistanceconfigured to adjust a phase of a signal generated by the inductor. 19.The signal adjuster of claim 18, wherein the phase is adjusted bychanging a resistor-capacitor time constant of a capacitor in serieswith a variable resistor.
 20. The signal adjuster of claim 19, furthercomprising a fixed resistor connected in parallel with the variableresistor.
 21. The signal adjuster of claim 18, wherein the resistance isprovided by a variable resistor.
 22. The signal adjuster of claim 18,wherein a second lead of the inductor is connected to ground.